Catalytic adsorbents for mercury removal from flue gas and methods of manufacture therefor

ABSTRACT

The present invention provides catalytic adsorbents formed from doping activated carbon with a dispersed halide salt. The catalytic adsorbents provided herein are stable and harmless at room temperature, yet allow for chemical adsorption at elevated temperatures typical of those for flue gas streams. The present invention also provides methods of manufacturing the doped activated carbon adsorbents.

TECHNICAL FIELD

The present invention relates generally to catalytic adsorbents for usein the removal of mercury from flue gas streams and methods ofmanufacturing such catalytic adsorbents.

BACKGROUND OF THE INVENTION

The toxicity of mercury to humans and the environment has long beenknown. It is known for example that mercury exposure can causeneurological damage in humans. A particularly devastating example of theharmful effects of mercury occurred in Minamata, Japan in the 1950'swhere organic mercury byproducts of acetaldehyde production weredischarged into the local bay. The byproducts were consumed andmetabolized by fish. By consuming fish in the bay, wide spreadneurological damage and birth defects among the local population werereported.

Coals used for generating electric power often contain about 0.1 ppmmercury. In the United States alone, about 50 tons of mercury aredischarged as vapor in stack gas every year. Through chemical andbiological processes, this mercury can become concentrated in fish bymany thousand fold, thereby entering human food supplies at harmfullevels.

The effort to remove trace mercury from air, water, natural gas, andother industrial streams has a long history, however; removing mercuryfrom coal burning flue gas streams is a very different problem.

Prior art techniques for removing mercury from air or hydrocarbons atroom temperature generally have limited relevance to removing mercuryfrom flue gas streams. Mercury has a high atomic weight and adsorptiontemperature is a significant issue. At room temperature, the dispersioninteraction with carbon is sufficient to immobilize mercury atoms. Atabout 300° F. (the temperature of many flue gas streams), however,physical adsorption is no longer able to hold down the volatileelemental mercury.

In addition, sufficient contact time with rapidly moving flue gasstreams is another issue for mercury removal. The total time for fluegas, from generation by combustion to exit through the stack, is oftenless than 10 seconds. Either as injected powder, where adsorbent flyamid flue gas is for about 2 seconds, or as filter cake on bags in a baghouse, the contact time between flue gas and activated carbon capturedby the filter is less than one second.

The demand on reactivity and reaction kinetics by flue gas cleaning cannot be properly tested by conventional packed beds. Conventional packedbeds are insufficient for flue gas cleaning because the volume of fluegas is so large, the cost for compressing it to push it through a packedbed is prohibitive.

Further issues relating to the removal of mercury from flue gas includethe small, yet potentially toxic, concentration levels of mercury in theflue gas streams. The concentration of mercury in flue gas streams is inμg/m³ whereas the concentration of mercury in many other industrialprocesses is on the order of mg/m³. Much early work considered effluentscontaining mercury in the 5 μg/m³ range (that is not much lower than theinitial concentration of mercury in the flue gas) as fully purified.

Above all, prior art techniques consider the adsorption of mercury as anevent between the adsorbent and the mercury. While this is true in airor hydrocarbon streams at room temperature, flue gas contains highlypolar and reactive components that can play both an interfering andenabling role for mercury removal. One model composition used for fluegas contains about: 6% O₂, 12% CO₂, 8% H₂O, 1600 ppm SO₂, 400 ppm NO, 50ppm HCl, 20 ppm NO₂, and 12 μg/m³ elemental Hg.

Prior art attempts to remove mercury from flue gas have included varioustechniques. One approach has focused on adding halogen salts into coalprior to combustion such that the combustion process generates hydrogenhalide gases and then injecting powder carbon downstream into the fluegas at a lower temperature. Some mercury is captured by interactionbetween the hydrogen halide gases, activated carbon and mercury. Anotherapproach has been to add hydrogen halides or elemental halogen togetherwith activated carbon to a lower temperature flue gas.

U.S. Pat. No. 1,984,164 to Karlsruhe proposes carbon or silica gel orother adsorbents impregnated with elementary halogen for removal ofmercury from room air. Other prior art attempts have included addinghalide salts to coal before combustion since these salts are known to bevery stable. The combustion process oxidizes halides to halogen andfurther reacts with hydrogen to yield hydrogen halides. For example,U.S. Pat. No. 5,435,980 to Felsvang et al. suggest adding chloride or achlorine containing material into the coal before or during combustionor adding HCl into flue gas upstream of or in the drying-absorptionzone.

U.S. Patent Application No. 2004/0003716 Al to Nelson, Jr. discloses amethod for removing mercury and mercury containing compounds fromcombustion gas by injecting an adsorbent into the flue stream. Thesorbent is prepared by treating a carbonaceous substrate with a brominecontaining gas. Bromine gas is known to be highly toxic by inhalation,ingestion or skin contact. HBr is also known to be corrosive. Inaddition, bromine and HBr compounds are reactive and can easily be addedonto alkenes. Further, bromine is reactive with aromatics.

U.S. Pat. No. 6,533,842 B1 to Maes et al. disclose powder adsorbentswhich contain about 40% carbon, 40% calcium hydroxide, 10% cupricchloride and 10% KI₃ impregnated carbon to remove mercury from a hightemperature, high moisture gas stream.

In December 2000, the United States Environmental Protection Agency(EPA) made its regulatory decision that mercury emissions fromcoal-fired electric generating plants need to be controlled.

In the field of the mercury removal from flue gas streams, it wouldtherefore be desirable to provide adsorbents having improved adsorbentcharacteristics in the flue gas temperature range and that can beeconomically and efficiently manufactured.

BRIEF SUMMARY OF THE INVENTION

The present invention provides catalytic adsorbents in which a halidesalt is dispersed on activated carbon and the oxidation catalyticactivity of the activated carbon promotes the formation of mercuryhalide. At the same time, the adsorbent qualities of activated carbonretain the mercury halides thus formed. The present invention recognizesthat while the halide salts are stable and harmless at room temperature,these doped activated carbon compounds form mercury halogen compounds atelevated temperatures typical of those found in flue gas streams, and inthe presence of reactive components typical of flue gas. These mercuryhalogen compounds are retained on the surface of the activated carbon.Moreover, the increased adsorbent capacity and faster rate of adsorptionresult in a need for smaller quantities of adsorbent relative to anundoped activated carbon formed from the same starting material.

A catalytic adsorbent composition for removal of mercury from a flue gasstream thus includes an activated carbon having a dopant (i.e, a halidesalt) dispersed thereon. The cation of the dopant used for the halidesalt in accordance with the present invention can be an alkaline,alkaline earth, or transition metal (e.g., Na, Ca, Mg, Cu and K). Theanion involved can be bromide or chloride. Particularly preferreddopants include, but are not limited to, NaCl, CaCl₂, CuCl₂, CuBr₂,NaBr, KBr, CaBr₂ and MgBr₂.

The halide salt is inert with respect to mercury and the activatedcarbon at room temperature. At elevated temperatures (e.g., 200-570° F.)and in the presence of typical flue gas compositions, mercury halogencompounds are formed and retained on the activated carbon. While notintending to be bound by any theory, it is believed that any or all ofthe following or a combination of the following may occur. An oxidant(for example, oxygen form the flue gas or oxidant on the activatedcarbon) oxidizes the mercury and the anion of the dopant provides acounter ion for the mercury ion as oxidized by the oxidant.Alternatively, the oxidant oxidizes the anion in the salt and theoxidized anion in turn oxidizes the mercury to form a mercury halogencompound on the activated carbon. In addition or in the alternative,acidic gases present in the flue gas react with the dopant salt to yielda hydrogen halide. The hydrogen halide is then oxidized by an oxidantand yields a halogen compound. The halogen compound then reacts with themercury to form a mercury halogen compound that are then adsorbed by theactivated carbon.

The present invention also provides methods of manufacturing such dopedactivated carbon adsorbents that are both economical and safe. Thecatalytic adsorbents of the present invention can be made from a varietyof methods. In one embodiment, the catalytic adsorbents can be formed byplacing an activated carbon in an aqueous solution containing a halidesalt to form a mixture, stirring the mixture until a homogeneous slurryis formed and drying the activated carbon such that water from theaqueous solution evaporates and the halide salt is dispersed on thesurface of the activated carbon.

In another exemplary method of manufacture, the catalytic adsorbents canbe made by injecting a presoaked carbonaceous feedstock into a reactionchamber together with oxidizing gases such as air and/or steam. Thecarbonaceous feedstock and the oxidizing gases are injected into thereaction chamber under conditions and for a residence time sufficient toform a powder activated carbon having a dopant dispersed on the surfaceof the powder activated carbon. In this method, the reaction chamber canbe a batch type reactor such as a tube furnace or a reactor designed forcontinuous mode operation (e.g., a fluidized bed reactor). The dopant isformed of a cation selected from the group including an alkaline metal,an alkaline earth metal, and a transition metal (e.g, Na, K, Mg, Ca andCu) while the anion is selected from bromide and chloride. In someembodiments, the dopant may be selected from the group including: NaCl,KCl, CaCl₂, CuCl₂, CuBr₂, NaBr, KBr, CaBr₂ and MgBr₂.

The catalytic adsorbents of the present invention are suitable for usein the removal of mercury from a gas stream containing an oxidant and/oracidic gases at an elevated temperature such as a flue gas streamexiting a boiler or combustion process. In this process, the catalyticadsorbents of the present invention are injected into the flue gasstream for an in-flight mode of mercury capture. As discussed above, thedopant is inert with respect to the mercury at room temperature. At fluegas temperatures and in the presence of the activated carbon, oxidantand/or acidic gases, however, the dopant effectively removes mercuryfrom the flue gas stream. The mercury is retained on the activatedcarbon in the form of mercury halogen compounds and can be separatedfrom the flue gas stream together with the flyash.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention and theadvantages thereof, reference should be made to the following DetailedDescription taken in conjunction with the accompanying drawings inwhich:

FIG. 1 illustrates one embodiment for manufacturing catalytic adsorbentsin accordance with the present invention;

FIG. 2 illustrates a method of using the catalytic adsorbents inaccordance with the present invention;

FIGS. 3-6 illustrate graphs relating to Example 1;

FIGS. 7-12 illustrate graphs relating to Example 2;

FIG. 13 illustrates a graph relating to Example 3; and

FIGS. 14-20 illustrate graphs relating to Example 4.

Similar reference characters refer to similar parts throughout theseveral views of the drawings.

DETAILED DESCRIPTION

The present invention provides catalytic adsorbents suitable for use inthe removal of mercury from flue gas streams at elevated temperatures.The catalytic adsorbents of the present invention include compositionshaving an activated carbon with a dopant dispersed on the activatedcarbon. The dopant is a halide salt. The cation of the dopant can be analkaline, alkaline earth, or transition metal while the anion of thedopant can be bromide or chloride. The catalytic adsorbents of thepresent invention can be formed from a variety of methods.

The present invention also provides methods of using these compositionsfor mercury capture at elevated temperature in the presence of acidicgases and/or oxidative gases that are commonly found in flue gas streamsgenerated by coal burning.

The mercury capture action is a synergistic combination of components inthe adsorbent compositions, the flue gas stream as well as the flue gasstream temperature. Activated carbon doped with bromide salts may beparticularly preferred adsorbents as the bromide salts appear to requireless assistance from acidic and/or oxidative gases in the flue gasstream and appear to be particularly effective at removing mercury fromthe flue gas stream.

As discussed hereinabove, alkaline, alkaline earth and transition metalhalides are harmless salts and inert to mercury and activated carbon atroom temperature. At about 200°-570° F. (e.g., 270° F.), however, and inthe presence of acidic gases and/or oxidative gases of flue gas, thesedoped activated carbon compositions are capable of capturing mercurywith high efficiency. Unused halide salts remain in their salt form.

The catalytic adsorbents of the present invention also perform well influe gas streams generated by burning low chloride coal (e.g., PowderRiver Basin (PRB) coal from Wyoming) where current adsorbents such asFGD carbon do not function efficiently.

The present invention thus provides for halide salts to be dispersed onactivated carbon such that the salts retain their chemical inertness atroom temperature, but react with mercury in hot flue gas to yield nonvolatile mercury halide. More particularly, at temperatures in the rangeof about 200-570° F., and in the presence of acidic and/or oxidative gasfrom the flue gas, halide salts react with mercury and assist theactivated carbon to capture the mercury, which is present in very lowconcentrations in flue gas streams. The catalytic adsorbents of thepresent invention utilize the very fast kinetics at elevatedtemperatures to optimize both physical adsorption as well as chemicaladsorption. The reactivity of the halide salts as used herein is thus acooperative phenomenon.

As discussed hereinbelow, the catalytic adsorbents of the presentinvention can be made from a variety of methods. The adsorbents can bemade from commercially available powdered activated carbon (PAC) or fromraw carbonaceous material. Exemplary PACs suitable for use in theinvention include, but are not limited to, FGD (available from NoritAmerica, Inc.), ashless activated carbon powder made from purifiedpetroleum coke and carbon fiber powder made by carbonization of rayonfiber. It will be appreciated that other activated carbons can also beused in the present invention.

The catalytic adsorbents of the present invention can be made fromvarious techniques. In one embodiment of the invention, the adsorbentscan be manufactured by soaking activated carbon in an aqueous solutionof halide salts. This approach is an economical and safe processrelative to treating activated carbon with hydrogen halides or halogengases.

In this embodiment, the minimum amount of water necessary to make asolution of the salt is utilized. The cation of the dopant can be analkaline, alkaline earth, or transition metal. The anion involved can bebromide or chloride. Suitable salts for use in the invention thereforeinclude, but are not limited to, NaCl, CaCl₂, CuCl₂, CuBr₂, NaBr, KBr,CaBr₂ and MgBr₂. In some embodiments, KBr, NaBr or CaBr₂ may bepreferred and in some embodiments, NaBr or KBr may be the most preferredsalt.

The PAC, preferably in powder form, is placed in the aqueous solutionand the mixture is stirred until it becomes a homogeneous slurry andsuch that there is sufficient contact time between the salt solution andPAC that the salt solution becomes dispersed on the PAC. It will beappreciated by those skilled in the art that the PAC has porosity suchthat the solution and hence the halide salt will disperse into the PAC.

In this approach, the amount of salt necessary for the aqueous solutionis determined based on the amount of PAC and the ratio of the salt toPAC that is desired for a particular adsorbent (i.e., the dopant levelin the desired PAC determines the concentration of the salt solution).In some embodiments, the ratio of the dopant level to that of the PAC is1:10,000 to 30:100. In more preferred embodiments, the ratio of dopantto PAC is 1:1000 to 10:100 and in other embodiments, the ratio of dopantto PAC is 0.5:100 to 7:100.

The salt solution containing the PAC is allowed to soak and then allowedto sufficiently dry such that the PAC is free flowing. During this time,the water evaporates and the salt enters the pore volume of the PAC andbecomes dispersed on the surface of the PAC. After the PAC is dried, itis in powder form. It may be ground and passed through an appropriatesize desired mesh. While not to be construed as limiting, the PAC may bepassed through a 200 mesh. In this manner, the PAC can be used formercury removal at less than or equal to a 200 mesh material. It will beappreciated by those skilled in the art that the adsorbent can betreated for appropriate size depending on the intended use of theadsorbent. For example, smaller mesh (e.g., 400 mesh) may be desirablein some applications.

It is believed that the catalytic adsorbents of the present inventionwill perform well for mercury removal from flue gas streams at elevatedtemperatures given the dispersed salts on the surface of the PAC. Whilenot intending to be bound by any theory, it is believed that the salt isinert with respect to elemental mercury at room and high (i.e. in therange of combustion zone) temperatures. At elevated temperatures ofabout 200-570 F (for example, at about 270-300 F), however, and in thepresence of oxidative and/or acidic gases in the flue gas, and the dopedactivated carbon, mercury in the flue gas stream can be oxidized andeffectively removed therefrom.

An alternative method to soaking a PAC in an aqueous solution asdescribed above is to spray water droplets containing the desired halidesalt on the PAC in a manner such that the halide salts become dispersedas discussed above. Such an approach can be used in connection with theactivated char produced in commonly owned U.S. patent application Ser.No. ______ entitled “Production of Activated Char Using Hot Gas” to Boolet al., filed on even date herewith. The entire contents of U.S. patentapplication Ser. No. ______ are incorporated herein by reference.

An alternative method for manufacturing catalytic adsorbents suitablefor use in the present invention is shown in FIG. 1. In this embodiment,the catalytic adsorbents can be manufactured by presoaking aprepulvurized carbonaceous feedstock in an aqueous solution of analkaline, alkaline earth or transition halide salt. Alternatively, theprepulverized carbonaceous feedstock may be soaked in an alcohol (e.g.,ethanol) solution containing the alkaline, alkaline earth or transitionhalide salt. The presoaked feedstock in then exposed to an oxidizing gasmixture such as air and steam at an elevated temperature in a reactionchamber to produce catalytic adsorbents and an exhaust gas.

The final concentration of the catalytic adsorbent is determined as inthe prior embodiment (i.e. the ratio of the dopant to activated carbonis predetermined in order to determine the concentration of the saltsolution), except that in this embodiment, the loss of carbon due tocombustion in the reaction chamber must be taken into account. One cantherefore determine the concentration based on the yield of the finalproduct to account for the loss of carbon due to combustion.

As illustrated in FIG. 1, carbonaceous feedstock 16 is injected intoreaction chamber 10. In some embodiments, carbonaceous feedstock 16 isnot yet activated and can be selected from various types of feedstocksuch as coal or biomass materials. The feedstock can be prepulverized toan appropriate size, for example from about 5-200 microns.

The carbonaceous feedstock 16 is also presoaked as discussed above priorto injection into reaction chamber 10 with a solution containing thedesired halide salt. In this embodiment, the solution can be formed fromwater or ethanol, although water may be preferred.

Oxidizing gases 12 and 14 (e.g., air 12 and steam 14) are injected intoreaction chamber 10 simultaneous with or nearly simultaneous withcarbonaceous feedstock 16. Preferably, the steam is preheated and isinjected at a temperature of about 1800 F.

Reaction chamber 10 may be selected from a variety of reactors such assingle batch reactors where the feedstock is suspended on a filter mediaand reactant gases pass through the feedstock (e.g, a tube furnace) orcontinuous reactors whereupon the gas temperature, composition andfeedstock residence time can be controlled for optimal conditions (e.g.,a fluidized bed reactor). One type of a continuous process reactor maybe a Plow Mixer, available from Scott Equipment Company.

Heat for reaction chamber 10 can be provided by from various sources,for example, the reaction chamber can be electrically heated or heatedby a flame. Alternatively or in addition to such heat, reaction chamber10 may be heated from the temperature of the feedstock and/or steam. Itwill be appreciated by those skilled in the art that the desiredtemperature within the reaction chamber depends on several factors,including temperature of the air and/or oxidizing gases, amount ofoxygen, stoichiometric ratio of oxygen to feedstock and/or temperatureof the feedstock. The heat may be provided from any source so long as itis sufficient to generate flue gas 18 and adsorbent 19. Typically, thetemperature within the furnace will be between about 1450-2700° F., andmore preferably between about 1650-2200° F. When the stochiometric ratioof oxygen to feedstock is greater than one, the contact time between theoxidizing gas and the feedstock becomes more significant because more ofthe feedstock will be consumed and therefore impact product yield. Whenthe stoichiometric ratio is less than one, the contact time will be lesscritical.

The residence time of the carbonaceous feedstock 16, reactive oxidizinggases (such as air 12 and steam 14) within reaction chamber 10 is longenough such that flue gas 18 and adsorbent 19 are generated withinchamber 10. The residence time of the carbon is independent of the gasand can be independently controlled. This can be significant becausesufficient time is necessary to devolatilize and partially oxidize thefeedstock. While the residence time is short, it is important that it belong enough to adequately activate the carbon. In some embodiments, theresidence time may be on the order of minutes. It will be appreciatedthat if the residence time is too long or there is too much oxygen orsteam, adsorbent yield will be negatively impacted.

Adsorbent 19 is removed from reaction chamber 10 and is ready for use asa mercury removal adsorbent from flue gas streams at elevatedtemperatures. Flue gas 18 typically includes combustion gases such asCO₂, CO, N₂ and H₂O. Any unreacted, partially combusted (e.g., CO) orvolatile gases in gas stream 18 can be further combusted.

Yet another alternative embodiment for manufacturing catalyticadsorbents for use in accordance with the present invention can be foundin commonly owned U.S. patent application Ser. No. ______, entitled“Production of Activated Char Using Hot Gas” to Bool et al., filed oneven date herewith. The entire contents of U.S. patent application Ser.No. ______ are incorporated herein by reference.

In this embodiment, the feedstock is presoaked with an aqueous orethanol solution as discussed above. The presoaked feedstock is thentreated to produce activated char as discussed in commonly owned U.S.patent application Ser. No. ______, entitled “Production of ActivatedChar Using Hot Gas”.

Catalytic adsorbents of the present invention can also be formed by drymixing a prepulverized raw carbonaceous material with a halide saltpowder. In this embodiment, the raw carbonaceous material and halidesalt powder are mixed together in dry form. The mixture can then beinjected and processed as discussed hereinabove with regard to FIG. 1 oras shown in commonly owned U.S. patent application Ser. No. ______,entitled “Production of Activated Char Using Hot Gas”. The temperaturewithin the reaction zone will be at or above the melting point of thehalide salt such that the halide salt melts and wets the surface of thecarbonaceous material. Consequently, the salt can be dispersed in thecarbonaceous material.

Referring now to FIG. 2, an exemplary system for using the catalyticadsorbents of the present invention is shown. Flue gas 22 is formed as aresult of combustion in a furnace or boiler 20. While flue gas 22 canvary in composition and temperature, a typical composition can include:6% O₂, 12% CO₂, 8% H₂O, 1600 ppm SO₂, 400 ppm NO, 50 ppm HCl, 20 ppmNO₂, and 12 μg/m³ elemental Hg and can be in the temperature range ofabout 200-570 F. Catalytic adsorbent 30 a, which can be formed from anyof the methods described hereinabove, can be injected upstream ofparticulate collection device (PCD) 24. Particulate collection device 24is typically a baghouse or electrostatic precipitators (ESPs). Adsorbent30 a is injected into flue gas stream 22 upstream of PCD 24 such thatthere is sufficient residence time for the catalytic adsorbent tocapture and remove mercury from flue gas 22.

Particulates and adsorbent containing mercury are removed from PCD 24 bystream 28. Flue gas 26 thus contains less mercury than flue gas 22 andmay be sent to the stack.

In some embodiments, it may be desirable to inject the catalyticadsorbent into the flue gas downstream of the PCD. Such processes arecurrently being investigated by others.

As discussed above, it is believed that the catalytic adsorbents of thepresent invention will perform well for mercury removal from flue gasstreams at elevated temperatures given the dispersed salts on thesurface of the PAC.

EXAMPLES

As will be seen hereinbelow, physical adsorption of PAC at about 270° F.is not sufficient to retain elementary mercury without HCl as apromoter. In contrast, doped PAC function well without HCl; however, thepresence of HCl, O₂ and/or SO₂ function as promoters for a doped PAC.

In some examples, doped PAC were prepared by treating three types ofcommercially available PAC. In other examples, doped PAC was prepared byactivation of halide salt treated coal.

The first commercial PAC used is FGD carbon, available from NoritAmerica, Inc. It is made from lignite coal and contains about 30 weightpercent ashes. In powder form, it is widely tested and accepted as abench mark for activated carbon for mercury removal from flue gas. Thesecond PAC was ashless activated carbon available from Carbon Resource,Inc. It is typically made from purified petroleum pitch and contains atrace amount of ash. It is generally sold in bead form. For mercuryremoval in the following examples, it was ground, sieved and the −400mesh portion was used. The third PAC that was used was activated carbonfiber ACF-1300/200, also available from Carbon Resources, Inc. It ismade from rayon and typically received in cloth form. This material wasground and screened through 400 mesh sieve before use.

To prepare halide salt doped PAC from coal, coal was soaked in anaqueous or ethanol halide salt solution. The doped coals were activatedin a stream of oxygen, nitrogen and steam in temperature range of about1800° F.

Two tests were used to evaluate the adsorbents: a fixed bed test and aresidence chamber test. In the fixed bed test, the fixed bed consistedof 150 mg adsorbent supported on a quartz filter of about 63.5 mm indiameter. The details of the test setup are described in paperspublished by EERC, as published for example at the Mercury ControlTechnology R&D Program Review Meeting on Aug. 12-13, 2003 at Pittsburgh,Pa. Gas streams containing mercury as well as components of flue gaswere passed through the thin bed. The break through of mercury wasmonitored and spent adsorption beds were collected and analyzed.

In the residence time chamber test, a slip stream from a power plant atPleasant Prairie, Wis. was made to pass through chambers of differentlength. Adsorbent was injected at one end of the chamber to flight withthe flue gas stream. At the other end of the chamber, the adsorbent wasseparated from the flue gas stream and the cleaned flue gas was analyzedfor Hg content to determine the efficiency of the adsorbent. The chamberlength was used to determine the contact time between the flue gas andthe adsorbent. Details of the residence time chamber apparatus (designedby the Electric Power Research Institute (EPRI)) can be found inpublished papers (see e.g., “Assessment of Low Cost Novel Sorbents forCoal Fired Power Plant Mercury Control”, Combined Power Plant AirPollutant Control Mega Symposium (Washington, D.C., Aug. 30-Sep. 2,2004).

Example 1

This example demonstrates that at room temperature, undoped PAC is agood adsorbent for elemental mercury and a promoter would appear toprovide no additional benefit. At 270° F., however, physical adsorptionis overwhelmed by kinetic energy and adsorption by undoped PAC andwithout a promoter was inadequate.

Fixed bed tests were conducted on four samples in a stream whichcontained nitrogen and about 13 μg/m³ of elemental mercury. The testsconditions and results are summarized in Table 1 and FIGS. 3-6. Theundoped FGD carbon sample was tested at room temperature and achieved100% mercury removal for more than 15 hours with no sign of mercurybreakthrough. For samples tested at 270° F., all three types ofactivated carbon reached almost 100% breakthrough immediately (0%removal). TABLE 1 Test gases Sample Sample Test composition and Commentson Test Sample # Name treatment Temp sequence Results FGD As received 72 F. N₂ + Hg 100% Hg removal for carbon 15 hrs; no any sign ofbreakthrough 17297-89 FGD Vacuum 270 F. N₂ + Hg Breakthrough carbonactivated at occurred immediately 1100 F. 17343-13 Carbon 6 N HCl 270 F.N₂ + Hg Breakthrough fiber extraction and occurred heating at 1800 F.immediately. in N₂ 17297-99 Ashless 6 N HCl 270 F. N₂ + Hg Breakthroughcarbon extraction and occurred immediately heating at 1800 F. in N₂

6 N HCl extraction was used in Sample Numbers 17343-13 and 17297-99 toremove any trace ashes. Heating in N₂ at 1800° F. is intended to removeoxidizing species on the commercially obtained PAC. Neither treatmentchanged the adsorption behavior of the PAC.

Example 2

This example demonstrates how halide salts as a dopant alter the fluegas, mercury and carbon interaction so as to promote mercury adsorptionfrom the flue gas stream. In this Example, thin fixed beds of PACsamples were exposed to different gas mixtures in sequence. Allexperiments started with nitrogen and mercury (about 13μ gm/cubicmeter). Other components of the flue were added into the streamsequentially or in sequential combination toward a composition ofsynthetic flue gas, which is typified as: 6% O₂, 12% CO₂, 8% H₂O, 1600ppm SO₂, 400 ppm NO, 20 ppm NO₂, 50 ppm HCl, 12-14 μg/m³ Hg, with thebalance being N₂.

Two type of PAC (ashless and FGD) and three dopants (KBr, NaBr, andNaCl) were used in the experiments. Detail of the experiments aresummarized in Table 2. The breakthrough curves are given in FIGS. 7-12.TABLE 2 Test gases Sample Sample composition and Comments on Sample #Name Description sequence Test Results 17297-99 Ashless 6 N HClextraction 1. N₂ + Hg; Removed Hg carbon and heating at 2. 6% O₂ + 8%H₂O only after HCl 1800 F in N₂    added; was added to the 3. 1600 ppmSO2 test gas    added; 4. 50 ppm HCl    added 17297-99 Ashless 6 N HClextraction 1. N₂+; Hg; HCl promoted Hg carbon and heating at 2. 50 ppmHCl adsorption, SO₂ 1800 F in N₂    added; caused decline of 3. 1600 ppmSO₂ Hg removal    added 17343-15 KBr doped 15:100 ratio of 1. N₂ + Hg;Adsorbed Hg in Ashless KBr:Carbon 2. 6% O₂ added; N₂ stream. Both carbon3. 8% H₂O added; O₂ and SO₂ 4. 1600 ppm SO₂ promoted Hg    added removal17297-89 FGD carbon Vacuum activated 1. N₂ + Hg; Removed Hg at 1100° F.2. 8% H₂O + 50 ppm only after HCl    HCl added; was added to the 3. 6%O₂ added; test gas 4. Full Flue added 17297-93 NaBr doped 15:100 ratioof 1. N₂ + Hg; Adsorbed Hg in FGD NaBr:Carbon 2. 8% H₂O + 12% N₂ stream.Both    CO₂ + 6% O₂ O₂ and SO₂    added; promoted Hg 3. 1600 ppm SO₂removal    added; 4. Full Flue added 17297-91 NaCl doped 15:100 ratioof 1. N₂ + Hg; CO₂ and SO₂ FGD NaCl:Carbon 2. +8% H₂O + 12% CO2 + 6%were weak    O₂ added; promoters. The 3. 1600 ppm SO₂ presence of HCl   added; was important for 4. Full Flue added; Hg removal 5. —HCl added

Example 3

This example demonstrates that a physical adsorbent such as silica gel,doped with KBr, did not remove mercury from the flue gas.

The same thin fixed bed method as in Examples 1 and 2 was used in thisExample. The details of sample preparation, test conditions and resultsare given in Table 3 and FIG. 13. TABLE 3 Test gases Comments SampleSample composition and on Test Sample # Name Description sequenceResults 17297-69 KBr The weight 1. O₂ 6% + CO₂ The con- doped ratio of12% + H₂O 8% + SO₂ centration of silica KBr:Silica 1600 ppm + NO Hgreduction gel gel = 400 ppm + HCl was less 15:100 50 ppm + NO₂ than 10%.20 ppm + Hg 14 micro gram + N₂ (full synthetic flue)

Example 4

This example analyzed the effectiveness of various halide salts asdopants. Doped ashless carbons were tested by thin fixed bed methods asin Examples 1-3 in synthetic flue. The results are compared with undopedFGD.

The thin fixed bed test is to simulate the function of a bag house in apower plant. The efficiency of adsorbent is analyzed by the percent ofmercury removal from the flue gas. All doped samples reached highermercury removal than undoped FGD carbon. The results are given below inTable 4 below. The breakthrough curves are shown in FIGS. 14-20. TABLE 4Test gases Sample composition and Comments on Sample # Sample NameDescription sequence Test Results 17343- KCl doped 15:100 ratio of 1. O₂6% + CO₂ 12% + H₂O Best removal at 02D ashless carbon KCl:Carbon 8% +SO₂ about 95% level 1600 ppm + NO 400 ppm + HCl 50 ppm + NO₂ 20 ppm + Hg14 μg/m³ + N₂ (full flue) 17343- NaCl doped 15:100 ratio of 1. O₂ 6% +CO₂ 12% + H₂O Best removal at 01C ashless carbon NaCl:Carbon 8% + SO2about 93% level 1600 ppm + NO 400 ppm + HCl 50 ppm + NO₂ 20 ppm + Hg 14μg/m³ + N₂ (full flue) 17343- NaBr doped 15:100 ratio of 1. O₂ 6% + CO₂Best removal at 02A ashless carbon NaBr:Carbon 12% + H₂O 8% + SO2 about98% level 1600 ppm + NO 400 ppm + HCl 50 ppm + NO₂ 20 ppm + Hg 14μg/m³ + N₂ (full flue) 17297- KBr:Carbon = 15:100 15:100 ratio of 1. O₂6% + CO₂ Best removal at 75A KBr:Carbon 12% + H₂O 8% + SO2 about 100%level 1600 ppm + NO 400 ppm + HCl 50 ppm + NO₂ 20 ppm + Hg 14 μg/m³ + N₂(full flue) 17343- CaBr₂ doped 15:100 ratio of 1. O₂ 6% + CO₂ Bestremoval at 02B ashless carbon CaBr₂:Carbon 12% + H₂O 8% + SO2 about 100%level 1600 ppm + NO 400 ppm + HCl 50 ppm + NO₂ 20 ppm + Hg 14 μg/m³ + N₂(full flue) 17343- MgBr₂ doped 15:100 ratio of 1. O₂ 6% + CO₂ Bestremoval at 02C ashless carbon MgBr₂:Carbon 12% + H₂O 8% + SO2 about 95%level 1600 ppm + NO 400 ppm + HCl 50 ppm + NO₂ 20 ppm + Hg 14 μg/m³ + N₂(full flue) 17297-62 FGD carbon From Norit 1. O₂ 6% + CO₂ Best removalat Amercia, a 12% + H₂O 8% + SO2 about 90% level reference 1600 ppm + NO400 ppm + HCl 50 ppm + NO₂ 20 ppm + Hg 14 μg/m³ + N₂ (full flue)

Example 5

This example used a residence time chamber test to demonstrate theeffectiveness of bromide salt doped PAC in an “in flight adsorption” andthe quality of PAC made by direct activation of bromide salt doped coal.

Residence time chamber test. The residence time chamber used in thisExample was an EPRI 8-inch diameter tube setup as discussed above. Aslip stream of 30 acfm flue was taken out from a coal burning boilerduct for flow through this tube. Adsorbent is injected at one end of thetube. At each of the middle section and exit end of this tube, there areone outlet sampling tubes to allow measurement at two differentresidence times. The mercury concentrations were measured at the inletas well as the sampling outlets to determine the mercury removalefficiency of the adsorbents.

The residence time chamber simulates the situation of a plant which hasonly an electrostatic precipitator (ESP), therefore mercury removaldepends on inflight adsorption. Typical inflight time is about 2seconds. In the example, the sampling outlets allow about 2 and 4seconds of residence time. Three groups of adsorbents were tested. Thefirst group of samples were prepared by doping FGD PAC with an aqueousbromide salt solution. The second group of samples were prepared byactivation of halide salt doped coal in a tube furnace at 1650° F. to2000° F. in a stream containing, oxygen, nitrogen and water. The thirdgroup of samples were prepared by activation of halide salt doped coalby a burner as in commonly owned U.S. patent application Ser. No.______, entitled “Production of Activated Char Using Hot Gas” to Bool etal., filed on even date herewith, with or without further steamactivation at 1800° F.

Undoped FGD carbon samples were also tested to serve a as reference. Thetest results are given in Table 5. The percentage of Hg removal iscalculated by dividing outlet mercury concentration with the inletmercury concentration. Since there is no way to determine how muchmercury is removed by the cylinder wall, the reported number is the sumof inflight removal plus removal by wall effect. TABLE 5 InjectionOutlet % Hg % Hg Sample Sample rate Temp Inlet Hg Hg Removal RemovalSample # Name Description lb/mmacf (° F.) μg/Nm³ 2/4 sec (2 sec) (4 sec)17297- FGD/ FGD 5.8 300 9 1.75/0.9  81 90 22 KBr carbon doped with 7:100ratio of KBr:FGD 17297- FGD/ FGD 5.7 300 8.7 1.3/0.7 85 92 23 KBr/carbon CuBr2 with 6:1:100 ratio of KBr:CuBr₂:FGD FGD No 6 300 6.43.3/2.8 48 56 doping 17343- Activated 7:100 6 300 10.3 3.0/2.0 71 80 76PRB coal ratio of predoped CaBr₂:Coal. with Activated CaBr2 in tubefurnace 17343- Activated 7:100 6 300 9.5 2.1/1.5 78 84 77 PRB coal ratioof predoped NaBr:coal. with Activated NaBr in tube furnace 17343-Activated 5:100 6 300 9.9 2.8/1.8 72 82 83B PRB coal ratio of predoped(CaBr₂ with ½H₂O):coal. CaBr2 Activated In tube furnace 78B Activated7:100 6 300 9.8 3.5/2.2 64 78 PRB coal ratio of predoped KBr:coal. withKBr Activated in burner 42A-15- Activated 7:100 6 300 9 1.7/1.2 81 871000 PRB coal ratio of predoped KBr:coal. with KBr Activated in burner,then steamed at 1800 F. (15 min)

Example 6

The chemical form of dopant in PAC. Activation of bromide salt dopedcoal is at a temperature close to 1800° F. This raises the questionwhether the bromide salt retains its ionic form. Chemical analyses ofbromide salt doped coal before and after activation are shown in Table6. Bromide salt maintains its inert ionic form. This may be particularlyadvantageous because bromination of carbon can create unknown andundesirable organic bromide compounds. It is therefore desirable toavoid the formation of such compounds. TABLE 6 Ionic Total Sample Samplebromine bromine Na Sample # Name Description mmol/gm mmol/gm mmol/gm17343- NaBr NaBr:PRB coal = 5:100 0.43 0.48 0.17 85A doped PRB Beforecoal activation 17343- 17343-85A tube furnace 0.65 0.70 0.26 85B afterat 1800 F, activation purged with 10% O₂, 90% N₂ saturated with watervapor at 194 F. 17343- NaBr NaBr:PRB coal = 1:100 0.09 0.08 0.03 88Adoped PRB before coal activation 17343- 17343-88A tube furnace 0.11 0.120.05 88B after at 1800 F, activation purged with 10% O₂, 90% N₂saturated with water vapor at 194 F.

It should be appreciated by those skilled in the art that the specificembodiments disclosed above may be readily utilized as a basis formodifying or designing other structures for carrying out the samepurposes of the present invention. It should also be realized by thoseskilled in the art that such equivalent constructions do not depart fromthe spirit and scope of the invention as set forth in the appendedclaims.

1. A catalytic adsorbent composition for removing mercury from a fluegas stream at elevated temperatures, comprising: an activated carbonhaving a halide salt dispersed thereon, the halide salt having a cationand an anion.
 2. The composition of claim 1, wherein the cation isselected from the group comprising: an alkaline metal, an alkaline earthmetal, and a transition metal.
 3. The composition of claim 2, whereinthe cation is selected from the group consisting of: Na, Mg, Ca, Cu andK.
 4. The composition of claim 1, wherein the anion is selected from thegroup comprising: bromide and chloride.
 5. The composition of claim 1,wherein the halide salt is selected from the group consisting of: NaCl,KCl, CaCl₂, CuCl₂, CuBr₂, NaBr, KBr, CaBr₂, MgBr₂ and mixtures thereof.6. The composition of claim 5, wherein the halide salt is NaBr, KBr ormixtures thereof.
 7. A method of making a catalytic adsorbent for use inthe adsorption of mercury from flue gas streams at elevatedtemperatures, comprising: placing a powder activated carbon in anaqueous solution containing a halide salt having a cation and an anionto form a mixture; stirring the mixture until a homogeneous slurry isformed; drying the powder activated carbon such that water from theaqueous solution evaporates and the halide salt is dispersed on thesurface of the powder activated carbon.
 8. The method of claim 7,wherein the cation is selected from the group comprising: an alkalinemetal, an alkaline earth metal, and a transition metal.
 9. The method ofclaim 8, wherein the cation is selected from the group consisting of:Na, Mg, Ca, Cu and K.
 10. The method of claim 7, wherein the anion isselected from the group comprising: bromide and chloride.
 11. The methodof claim 7, wherein the halide salt is selected from the groupconsisting of: NaCl, CaCl₂, CuCl₂, CuBr₂, NaBr, KBr, CaBr₂, MgBr₂ andmixtures thereof.
 12. The method of claim 11, wherein the halide salt isNaBr, KBr or mixtures thereof.
 13. A method of making catalyticadsorbent for use in the adsorption of mercury from flue gas streams atelevated temperatures, comprising: injecting a presoaked carbonaceousfeedstock into a reaction chamber; and injecting at least one oxidizinggas into the reaction chamber; injecting steam into the reactionchamber, wherein the carbonaceous feedstock, the air and the steam areinjected into the reaction chamber under conditions and for a residencetime sufficient to form an activated carbon having a halide salt havinga cation and an anion dispersed on the surface of the activated carbon.14. The method of claim 13, wherein the oxidizing gas comprises: air,oxygen, steam, nitrogen or combinations thereof.
 15. The method of claim13, wherein the reaction chamber is a tube furnace.
 16. The method ofclaim 13, wherein the reaction chamber is a fluidized bed reactor. 17.The method of claim 13, wherein the cation is selected from the groupcomprising: an alkaline metal, an alkaline earth metal, and a transitionmetal.
 18. The method of claim 17, wherein the cation is selected fromthe group consisting of: Na, Ca, Cu and K.
 19. The method of claim 13,wherein the anion is selected from the group comprising: bromide andchloride.
 20. The method of claim 13, wherein the halide salt isselected from the group consisting of: NaCl, KCl, CaCl₂, CuCl₂, CuBr₂,NaBr, KBr, CaBr₂, MgBr₂ or mixtures thereof.
 21. The method of claim 19,wherein the halide salt is NaBr, KBr or mixtures thereof.
 22. A methodfor removing mercury from a gas stream at an elevated temperature, themethod comprising: injecting a catalytic adsorbent containing anactivated carbon and a dopant, the dopant having a cation and an anioninto the gas stream; adsorbing mercury onto the catalytic adsorbent; andremoving the mercury containing catalytic adsorbent from the gas stream.23. The method of claim 22, wherein the gas stream contains oxidativegas, acidic gas or a combination thereof.
 24. The method of claim 22,wherein the gas stream contains an inert gas.
 25. The method of claim24, wherein the inert gas comprises nitrogen.